The goal of this proposal is to identify, clone and characterize a nuclear DNA binding factor that collaborates with a second DNA binding protein, TEF-1, to control cell specific transcription of the cardiac troponin T gene (cTNT). The mechanisms that control cardiac specific gene expression are unknown but transcription of many cardiac specific genes is dependent upon DNA sequence elements known as MCAT elements. MCAT elements are known to be binding targets for members of the TEF-1 family of transcription factors. Some TEF-1 genes are preferentially expressed in cardiac tissue and homozygous knock-out of at least one TEF-1 gene is embryonic lethal due to cardiac dysgenesis. Recent experiments show that TEF-1 must bind cooperatively with a second factor (herein dubbed factor X) to control muscle specific transcription. Importantly, factor X appears to be required both for transcriptional activation in muscle cells as well as transcriptional repression in non-muscle cells. Thus, protein X may represent a protein (or family of proteins) that are involved in both positive and negative cell selective transcription. The investigators will identify X candidates via a series of molecular and functional criteria and then test candidates for their ability to cooperatively bind with TEF-1 to MCAT targets. Once cloned, X protein(s) will be analyzed in detail to understand the molecular mechanisms by which cardiac specific gene transcription is controlled.